Need Help Locating a Part for My 1990 Chevy Pickup

Just noticed the plug that goes into the side of clutch housing on my truck is missing. It's the infamous GETRAG trans. Tried to get it at a dealer but it is now longer available, listed as being discontinued. The part number is 15677601 and the dimensions on the plug are M24x2.0. If anyone knows of where I can get one or a substitute part, please let me know.